Generally, an optical image reading apparatus has a function to read an image information of an origional by guiding the reflected light image of the original to a photoelectric conversion means such as light receiving element through an optical scanning device including a mirror unit and a lens unit. It is a fundermental condition for such an apparatus to maintain the distance of the reflected light path as constant as possible in order for the apparatus to work stably, said path starting from the image reading point where the light is reflected by the original and ending to the light receiving surface of the photo-electric sensor such as Charge Coupled Device where the senser receives the reflected light. Such conditions may be accomplished by an action of an adjusting device in a manufacturing step of the apparatus. However, when the distance of the light path changes accidentally, for example, due to distortion of the housing of the apparatus by placing it on an uneven place in use, re-adjusting of the error distance is extremely difficult. Accordingly, in order to prevent the occurrence of the error distance of the optical path in use, the prior art device selects the housing made of a rigid material or requires a strict condition as to the place installed. The former idea causes the apparatus cost-increasing and the latter has a limit to the completeness by itself.